The gap between a truck tractor and a trailer connected to the tractor is needed to allow the articulated movements for steering and to accommodate road risings and dips while avoiding contact between the tractor and trailer. The gap, however, is a well-known source of aerodynamic drag on the vehicle combination. Many devices have been proposed to solve this problem, including U.S. Pat. No. 3,71,146 showing telescoping panels to close the gap; U.S. Pat. No. 6,991,281 showing a roof fairing and U.S. Pat. Nos. 6,846,035 and 5,078,448 showing side fairings to direct air flow around or past the gap; U.S. Pat. No. 3,945,677 disclosing a flexible bag-like device filling the gap; U.S. Pat. No. 3,697,120 showing flow guides to create a swirl in the gap; U.S. Pat. No. 4,257,640 disclosing a baffle to block air flow across the gap; and, U.S. Pat. No. 7,216,923 disclosing a device to inject air into the gap to increase the pressure in the gap.
One problem in fairings is that the relatively high pressure air coming off the fairing trailing edge of the fairing is drawn into the relatively lower pressure air in the gap, creating turbulence and increased drag.
The inventor has posited that if a boundary layer of the flow along the side of truck tractor can be bled off of the fairing into the gap, that is, divided from the main flow and directed into the gap, a higher pressure region can be created in the gap along with an accompanying lower pressure along the fairing. This would equalize the pressure at the trailing edge of the fairing and prevent the formation of turbulence.
The invention proposes a device to control the air flow boundary layer at the gap to prevent turbulent flow at the gap. The invention concerns a flow guiding device placed where a cab side fairing is conventionally positioned on the truck tractor, the flow guiding device shaped to divide an air flow over the truck tractor at a boundary layer, directing a portion of the air flow into the gap between the tractor and the trailer, and directing the remainder to flow past the trailer. The divided portion rejoins the main portion of the air flow in such a way that turbulence in the air gap is prevented or reduced.
According to the invention, a flow guiding device is disposed at the rear of a truck tractor cab to extend vertically at the front portion of the gap between the truck tractor and a trailer. The device is spaced laterally from the truck tractor to provide a space at the forward end of the device between the device and the side of the truck tractor. The device extends longitudinally (parallel to the long axis of the truck) from a point adjacent the truck tractor to a point rearward of the truck tractor along the gap. The device divides the air flow, directing a relatively higher pressure stream into the gap side of the device, which causes the pressure at the trailing end of the device to equalize when the flows rejoin, reducing turbulence in the gap, and promoting a lower drag flow directed to the trailer. The longitudinal dimension of the device is sufficient to produce the flow effect from the trailing edge of reduced turbulence in the gap and a directed flow to the trailer.
An air guiding device according to the invention has two opposing surfaces shaped so that flow on one surface is slower than flow on the other surface. The device may be shaped as an airfoil, having a concave surface and an opposite convex surface. Alternatively, the device may have a convex surface and an opposite flat surface.
The device is mounted by brackets to the truck cab, the number of brackets being selected to prevent excessive flexing or bending of the device.
The device may be mounted with a convex surface facing the gap, and oriented to bleed off a layer of air flow into the gap, so that the outer and inner flows (relative to the device) rejoin without producing turbulence. Alternatively, the device may be mounted with the concave or flat surface facing the gap. It is expected that the orientation of the device relative to the gap, that is, the angle the device forms with the longitudinal direction, will influence the flow dividing and pressure equalizing effect.
The device may be mounted to be freely pivotable along its vertical axis to allow for self-adjustment. Alternatively, the orientation can be controlled by actuators responsive to electronic pressure sensing devices monitoring pressure on the outer and inner sides of the device. A controlled system can continuously adjust the device for optimum air flow.
A preferred embodiment of the invention includes an airfoil shaped device positioned to divide the flow and which device can be tuned to equalize the pressure in the outer and inner flows, as well as direct a slip stream to the trailer. Further, a preferred embodiment of the invention is a device that dynamically adjusts to air flow conditions to optimize air flow across the gap, and creates a substantially laminar flow directed to the leading edge of the trailer.
An airfoil device according to the invention may take any of a number of shapes capable of producing a flow effect to equalize pressure in an air flow downstream of the airfoil device, including, but not limited to, cambered, symmetrical, non-symmetrical, and circular arc, among others.
The device may include vortex generators formed or disposed at the trailing edge of the device to generate vortices in the flow directed to the trailer. Vortices in the air flow on a surface such as the trailer outer surface reduce surface friction.
According to another embodiment, the device of the invention is a vented fairing, a conventional fairing shaped or mounted on the truck tractor to bleed a layer of air into the tractor-trailer gap from the air flow and allow the divided flows to rejoin at a trailing edge.